Various mechanisms exist for reducing power consumption of a platform. Standard technology for power management is specified in Advanced Configuration and Power Interface (ACPI) version 2.0, which may be found on the public Internet at URL www-acpi-info (note that periods have been replaced with dashes to avoid unintentional hyperlinks in this document). ACPI is the standard most computer systems currently use for power management and is used to describe how the system looks to the operating system. Power management capabilities enable a platform, both at component and system level, to change its operating state to use less power and to resume normal operations. These modes of operation are often called sleep and wake modes. A number of events may trigger a platform to transition to low power state or to transition from a low power state to a more active state. For instance, moving a mouse, pressing a key on the keyboard or receiving a message may cause a platform to transition to an active state from a sleep, or low power, state.
The role of firmware in power management has been typically that the firmware can communicate with the hardware. The firmware may initiate stand-by or wake events. Often the policy of whether an event should wake or sleep the platform is dictated by the operating system (OS). The firmware will comply with the OS policy.
A typical scenario in existing systems is described herein. The platform is powered up. The firmware initializes the subsystem and eventually loads the OS. While the OS is running, at some point in time, a sleep event is triggered in the platform, perhaps due to time, or inactivity, or user-selection. The OS may trigger a system management interrupt (SMI) to cause a sleep event. The firmware acts upon the SMI and recognizes that the platform is to be put into sleep mode. The firmware initiates sleep mode. The chipset for the platform is programmed to recognize certain actions for triggering wake events. When an event causing a wake event is recognized, the firmware operates once again to wake the platform. The waking of a platform may be similar to boot up, but varies according to ACPI standards. Memory contents should still be intact and the operational context should be maintained. If the processor was in sleep mode, upon waking, the processor resets and re-launches the firmware. The firmware can detect that it is not in boot up mode, because in some sleep modes, memory remains intact. Thus, the firmware executes a process dedicated to waking the platform rather than rebooting it. The firmware will not reinitialize memory, for instance. Once awakened, the firmware transitions control to the operating system.